Summary The Transwest Air Limited, Beech 99A aircraft, C-FDYF, serialNo.U-110, was on a scheduled flight from Saskatoon, Saskatchewan, to Prince Albert, Saskatchewan, with two pilots and four passengers on board. The aircraft was approximately 4000feet above sea level when the crew selected the flaps for the approach to Prince Albert. A bang was heard from the rear of the fuselage. The aircraft commenced an uncommanded pitch-up to a near-vertical attitude, then stalled, nosed over, and began a spin to the left. The crew countered the spin but the aircraft continued to descend in a near-vertical dive. Through the application of full-up elevator and the manipulation of power settings, the pilots were able to bring the aircraft to a near-horizontal attitude. The crew extended the landing gear and issued a Mayday call, indicating that they were conducting a forced landing. The aircraft struck a knoll, tearing away the belly cargo pod and the landing gear. The aircraft bounced into the air and travelled approximately 180metres, then contacted a barbed-wire fence and slid to a stop approximately 600metres from the initial impact point. The crew and passengers suffered serious but non-life-threatening injuries. All of the occupants exited through the main cabin door at the rear of the aircraft. The accident occurred during daylight hours at 1802 central standard time. Ce rapport est galement disponible en franais. Other Factual Information Records show that the pilots were certified and qualified in accordance with current regulations. All required flight control checks had been completed, and, at the time of the occurrence, the aircraft was being operated with normal aerodynamic flight loads. The reported weather for Prince Albert at 1800 central standard time1 was as follows: wind from 120at 14gusting to 21knots, visibility 15statute miles, a few clouds at 6000and at 9000feet, and temperature 21C. The aircraft was equipped with a Fairchild model A100 cockpit voice recorder (CVR). The CVR was removed from the aircraft and forwarded to the Transportation Safety Board of Canada (TSB) Engineering Branch for examination. The crew activated the Narco ELT-10 prior to impact, and the emergency locating signal was successfully transmitted throughout the occurrence and after the aircraft came to rest. After the accident, inspection of the horizontal stabilizer trim control system revealed that the stabilizer trim actuator had fallen from the upper airframe mounting structure, allowing the stabilizer to move freely in flight. The mounting bolts had been installed through the airframe brackets but not through the mounting lugs of the trim actuator. Contact marks on the mounting lug assemblies, marks on the channel and plate assemblies, and worn rivets indicated that the upper mounting lugs had been positioned ahead of the normal mounting location. Placed in this manner, the spherical bearings of the lugs were positioned slightly above and in contact with the plate-to-channel attachment rivets. Marks on the rear of each mounting lug confirmed that the mounting bolts were installed behind the mounting lugs, trapping the mounting lug assemblies between the rivets and the bolt shanks. Positioned in this manner, the mounting lugs are approximately inch ahead of their appropriate location (seeAppendixA). The occurrence flight was the 12th flight after the completion of the continuing airworthiness program heavy-maintenance inspections that involved disassembly, inspection, and re-assembly of the aircraft wings and tail section. During this process, it was decided that the horizontal stabilizer trim actuator required replacement. A replacement unit was installed, and the unit was tested for functionality and adjusted. The procedures involved multiple applications of stabilizer trimming through all ranges of its operation. The airframe mounting structure for the horizontal stabilizer trim actuator consists of four channel-and-plate assemblies that are aligned fore-and-aft and installed between a former, located at station418.900, and a bulkhead located at station 428.335, in the upper section of the aircraft tail. To provide a bolt hole through which close-tolerance mounting bolts are installed, a plate is rivetted to each channel. There are two upper mounting lugs on the horizontal stabilizer trim actuator; both mounting lugs include a spherical bearing. To attach the trim actuator to the upper airframe structure, a separate high-tolerance bolt is installed through the holes in each set of paired channels and through the spherical bearing for each of the mounting lugs. The installer is on his knees, leaned forward and holding the actuator (approximately 10pounds) in place, above and ahead. The mounting lugs are hidden from view between their respective channels and the bolts are installed by feel. At any time in the process, the spherical bearings may rotate slightly, making it difficult to insert the close-tolerance bolts. The normal procedure is to insert a bolt through one of the channel pairs and the mounting lug, suspending the weight of the actuator from the airframe mounting structure (seeFigures1and2). Figure1. Horizontal stabilizer trim actuator mount structure Figure2. Horizontal stabilizer trim actuator On the ground, the weight of the elevator causes the leading edge of the horizontal stabilizer to pivot upwards applying a compressive load through the stabilizer trim actuator to the airframe mounting structure. During flight, the actuator experiences both compressive and tensile loads. The Beech99 Airliner Series Maintenance Manual, section27-40-00, specifies the procedure for accomplishing horizontal stabilizer trim actuator load testing. This procedure is designed to assess the proper operation of the internal components of the actuator. The trim actuator is run through its range of travel, with weights applied to the leading edge and then to the trailing edge of the horizontal stabilizer and elevator assembly, to assess smoothness and efficiency of operation. As an alternative to the load test, the actuator may be replaced by a new or overhauled actuator. In this instance, since the actuator was replaced with an overhauled unit, this procedure was not accomplished. The aircraft maintenance engineer (AME) who installed the horizontal stabilizer trim actuator had worked approximately 32 years in aviation and was experienced with maintenance procedures on Beech aircraft. Although the installer had not previously installed a horizontal stabilizer trim actuator on a Beech99A aircraft, he referred to the maintenance manual during the process. The Beech99 Airliner Series Maintenance Manual, section27-40-00, provides instructions for the horizontal stabilizer actuator's installation. Sub-paragraphc. directs the installer to secure the actuator to the fuselage cross member with two upper bolts and one of the lower bolts Also in that sub-paragraph, there is a double-indented, side-barred, caution message, which states, Install the two washers on each of the two upper bolts outside of the fuselage brackets as shown in figure201A. During the installation, the AME placed washers outboard of the brackets/channels, as prescribed. To prevent distortion of the mounting channels during tightening of the bolt, the AME attempted to install an additional washer between the spherical bearing and the mounting channel. After the occurrence, the washer was found loose in the belly of the aircraft. With the trim actuator installed between the airframe mounting channels, the area behind the mounting lugs is hidden from view. When viewed from the front, the heads and nuts of the bolts line up outside the channels, but it is difficult to assess whether the bolts pass through or behind the lugs. When the bolts have been installed, the mounting lugs are hidden behind the channel assemblies. There are no alignment marks on the sides of the actuator or on the airframe channels to identify the correct positioning of the actuator. At the completion of the maintenance work, the floor supervisor, an AME with more than 25years of aviation experience, completed a dual inspection and certification for the horizontal stabilizer actuator and other flight controls affected during maintenance. In addition, because of the extensive amount of work accomplished, the floor supervisor had other qualified AMEs inspect the flight controls prior to releasing the aircraft for a test flight. Transport Canada published Airworthiness Notice (AN) C010, Edition2, dated 10October2001, on the inspection of control systems. ANC010 explains the regulations applicable to the maintenance of engine and flight controls and outlines the applicable standards for control system maintenance, especially for the second independent control check of control systems after maintenance. Both the installing AME and the floor supervisor had previously readANC010. The TSB investigated a similar occurrence in June 1999 (TSBReportNo.A99H0002) where a Beech King AirA100 aircraft sustained a similar detachment of the horizontal stabilizer trim actuator just after takeoff. Prior to that occurrence, the upper attachment bolts for the horizontal stabilizer trim actuator had been installed through the airframe mounting structure but not through the upper mounting lugs of the actuator. Information gathered during that investigation indicated that the tightening of the attachment bolts during installation had squeezed the ends of the actuators to the attachment points on the airframe. The horizontal stabilizer trim actuators and the airframe attachment structure are identical for the B99and B100model aircraft.